The present invention relates to an appliance and to a method for preparing a froth from a food liquid such as milk and is intended, for example, to be used in the context of the preparation of beverages such as cappuccinos, milky coffees, café latte and macchiato, hot chocolate and other speciality hot beverages.
Speciality beverages in which at least a portion is made up of froth are increasingly becoming all the rage. The best-known beverage of this type is a coffee of the cappuccino type. It comprises a liquid portion consisting of coffee topped by a layer of frothed milk which, because of its very much lower density, floats atop the surface of the liquid. In general, preparing one takes time, manipulation operations and cleaning.
The most customary way of preparing a milk-based froth is to pour the desired amount of milk into the container, immerse a steam outlet pipe from a coffee machine in the container, agitating it up and down to introduce the air needed to form the froth.
A first disadvantage with this method is that it entails having a coffee machine equipped with a steam outlet, and not all machines are.
Another disadvantage lies in the fact that it is generally not possible to prepare the froth at the same time as preparing the coffee, unless a complex and expensive machine with two separate fluid circuits, one for extracting the coffee and the other for producing steam, is available.
Another disadvantage associated with the use of coffee machines with a steam outlet stems from the fact that the system for heating the water in these machines, most often a thermoblock, has to be kept constantly switched on in order to produce the steam without having to wait. These machines therefore consume large amounts of power, which makes them not very economical to operate. It must also be noted that steam pipes soon become scaled up when the water used is hard water and this entails regular maintenance in order to keep them in good condition.
Another disadvantage with this type of machine stems from the fact that the quality of the froth is dependent on the skill of the user which means that these machines do not allow the froth obtained to have reproducible properties and a uniform quality.
Another disadvantage stems from the fact that the pipes in contact with the milk are difficult and not very practical to clean.
There also exists mechanical stirring appliances which are usually intended for domestic use for beating froth from more or less viscous food products such as eggs, ice, juices or the like. The problem with these appliances is of several natures which means that they are ill-suited to producing froth from a milk-based liquid for making a beverage. One disadvantage, for example, stems from the fact that these appliances stir a liquid or a paste in the cold state, considerably limiting their potential use. In addition, milk does not froth as convincingly when cold or at ambient temperature.
Another disadvantage stems from the fact that these appliances are ill-suited to frothing the microbiologically sensitive liquids such as milk. Regular cleaning of the tank of the appliance needs to be envisaged in order to remove any solid food residue. In addition, heating the milk has a tendency to increase the extent to which cooked or burnt proteins are deposited on and adhere to the surfaces. The existing appliances are not, for the most part, well suited to reducing the encrustation of this solid residue, making cleaning troublesome.
These appliances also have a stirring and driving mechanism which is fixed and intrudes into the tank, and this presents several disadvantages: the removal/refitting time is not insignificant, they have a tendency to become soiled more quickly, they entail additional cost as a result of the multiplicity of components, and the stirring means are difficult to clean.
Another disadvantage with these appliances stems from the fact that frothing in these appliances is not optimal for reasons associated with the arrangement of the stirring means. Some appliances have a stirring means positioned in the middle of the tank. Such an arrangement is not efficient in quickly converting a volume of milk-based liquid into a rich froth.
Other appliances have several stirrers. These arrangements are generally mechanical with gearing means needed to coordinate the rotation of the stirrers, and this makes the system intrusive, and therefore less hygienic, increases the number of parts and makes cleaning a more painstaking operation.
U.S. Pat. No. 6,318,247 relates to an appliance for preparing hot beverages or food with stirring such as hot chocolate, for example. The appliance comprises a system for driving a stirrer of the magnetic effect type. It does, however, have several disadvantages. Firstly, in such an appliance, the liquid or froth is stirred coaxially with respect to the central axis of the tank, and this causes a circulation in which some layers of liquid or froth, particularly the peripheral layers, are not stirred as extensively as others, particularly the layers near the middle, because of the centrifuging effect produced by the stirrer. Such a circulation is therefore not suited to producing a froth of sufficient quality or to reducing the time needed to produce this froth. In addition, the structure of the appliance is not hygienic for treating a milk-based liquid and cleaning is not made any easier by such a construction.
Other devices for stirring food products which have more or less the same disadvantages are described in patent documents WO 2004/043213 A1 or DE 196 24 648 A1.
Other stirring systems of the magnetic engagement type are described in U.S. Pat. Nos. 2,932,493, 4,537,332 and 6,712,497 and in German patent application DE 1 131 372, but none of these provide solutions suited to producing a quality froth in a short time and hygienically.
German Utility model DE 89 15 094 relates to a refrigerated pot for dispensing a milk-based beverage. The pot comprises refrigerating means and forced-convection means in the form of a magnetic rotor. Such a device is not suited to converting the liquid into a froth but simply keeps the liquid uniformly at the refrigeration temperature for preservation.
U.S. Pat. No. 3,356,349 relates to a stirring device comprising a heated tank, magnetic driving mechanism positioned under the tank for driving a hub located in the middle of the tank. The hub is associated with a stirring element which is offset with respect to the hub, causing the element an oscillatory movement. Such a principle of stirring may create a loss in magnetic coupling because of the high moment of inertia generated by the offset position of the stirring means with respect to the axis of rotation of the driving mechanism. This problem increases with increasing speed. Such a stirring device can therefore work to stir certain liquids or other substances at low speed but may prove ill-suited to converting liquid into a froth at sufficiently high speed. Incidentally, the document does not mention the conversion of a liquid into a froth using this device.
Thus, there is a need for a device that overcomes the disadvantages of the prior art, and this is now provided by the present invention.